Conventional segmented seals generally comprise one or more arcuate segments that surround a rotatable or reciprocable shaft with the seal elements held by a fluid pressure and/or light spring force in intimate contact with both the shaft and a stationary surface on the seal housing extending radially of the shaft axis. The seal segments are prevented from rotating about the shaft by one or more stationary pins or lugs located on the radial surface of the housing and loosely engaging holes or slots in the segments. Details of construction vary widely to suit particular applications and to follow the design practices of various manufacturers. However, the use of rigidly fixed anti-rotation features seems to be virtually universal.
The seal segments are usually held together in ring form by a surrounding "garter" spring which also holds the segments in light but intimate contact with a rotating or reciprocating shaft. The seal segments are usually proportioned to leave extremely small circumferential gaps between abutting segments to assure radial contact with the rotating shaft surface and to permit a "wearing in" process to occur. Seal segments may vary from a single one piece type with one circumferential gap to quite complex sets of two, three or more segments. However, all known segmented seal constructions share the common principle of limited radial freedom with fixed circumferential restraint which requires that the restraining features must be very accurately located relative to the rotating shaft axis.
The anti-rotation pins or lugs usually employed in segmented seals as described above require a prohibitively high order of accuracy in the manufacture of the related elements. Further, the stationary seal housing must be very accurately centered relative to the shaft axis if radial loads on the seal segments are to be avoided. The relatively loose engagement of the anti-rotation pins or lugs in their corresponding holes or slots in the ring segments is frequently cited by seal manufacturers as the means of accommodating displacements of the stationary anti-rotation features relative to the shaft axis. However, a critical examination of the geometric principles involved demonstrates that elimination of radial load occurs only when the pins or lugs are precisely located in only one position. Lateral displacement from this position will generally result in only one of the anti-rotation pins providing the anti-rotation torque with a resultant lateral force being generated on the sealing surface to provide the required reaction to maintain the force balance equilibrium of the seal segment. This produces excessive and uneven wear because of the continuous lateral force on the circular sealing surface of the seal segment due to a "camming" action. This force can be relieved only by seal segment wear and in an extreme case a seal could be literally "worn out" before it is "worn in".
In most cases the positional errors due to manufacturing and assembly are small, and generally satisfactory results are achieved, but the predictable life of a seal is uncertain and the variables described above may produce unacceptable results where maximum and dependable seal life is essential.